Electric incandescent lamp and method of manufacture



June 3, 1969 B. E. SHANKS 3,448,321

' ELECTRIC INCANDESCENT LAMP AND METHOD OF MANUFACTURE I Filed Oct. 2. 1967 23 X my J8 lnvenfior: Bruce E. S banks by gas A104,,

United States Patent US. Cl. 313-315 4 Claims ABSTRACT OF THE DISCLOSURE In a quartz incandescent lamp having integral portions of a fine incandesci-ble tungsten filament hermetically sealed through the quartz, the tungsten filament is coiled on a molybdenum mandrel, respective ends of the coil and mandrel assembly are coated with an acid resistant material, the portion of the mandrel between those end portions is dissolved to leave relatively massive legs of coil and mandrel wire, the coating is removed from the legs, the filament is disposed within a quartz envelope with the legs extending from an open end or ends of the envelope, the open end of the envelope is heated above the softening point while portions of the filament contiguous to the legs are highly heated and maintained free from oxide coating or adsorbed gases, and the softened envelope is compressed to form a hermetic seal with the said portions of the filament contiguous to the legs, and to also embed portions of said legs which serve as non-incandescible lead-in Wires.

BACKGROUND OF THE INVENTION Field of the invention The invention relates generally to electric incandescent lamps and more particularly to tungsten filament incandescent lamps having envelopes of essentially fused silica or quartz glass. Still more particularly, the invention relates to the construction and manufacture of a hermetic seal and current supply connections to the filament.

Description of the prior art Although not necessarily limited thereto, the invention has particular application to, and will be described with reference to, incandescent lamps of the halogen regenerative cycle type wherein a tungsten filament is hermetically sealed in a compact envelope of fused silica which is heated to a relatively high temperature and which contains an inert gas filling together with a small quantity of a halogen, particularly iodine or bromine or compounds thereof. During operation of the lamp, tungsten vaporized from the filament and normally deposited as a dark film on the envelope Walls, reacts with the halogen to form a compound which migrates back to the vicinity of the filament where it is broken down to release the tungsten which is redeposited on the filament and to also release the halogen which is then free to repeat the cyclic reaction.

In view of the very low coeificient of expansion of fused silica or quartz, the provision of a hermetic seal of the current lead-in conductors requires the use of a molybdenum foil of small dimensional tolerances in order to obtain a tight hermetic seal in the quartz and to provide the necessary current carrying capacity. In some cases, the foil is a separate piece of metal having a lead wire welded to each end thereof, one wire extending to the exterior of the lamp envelope and the other to the interior where it is connected to one end of the filament. In other cases, the foil is a rolled portion of a molybdenum wire. In either case the foil is a relatively costly part of the lamp. Such foil type structures are used in lamps of the double-ended type wherein a tubular quartz envelope contains an axially ice extending filament and has pinch seals at respective ends thereof in which are hermetically sealed a foil and lead wires connected to respective ends of the filament. Such seals are also used in lamps of the single-ended type wherein the quartz envelope has a pinch seal at one end thereof through which are sealed both foil type leadin structures for respective ends of the filament.

A structure and method of manufacture wherein the foil is eliminated is disclosed and claimed in applications Ser. No. 601,930 to R. C. Millikan and Ser. No. 601,927 to R. C. Millikan and L. A. Osburg, both filed Dec. 15, 1966, and the disclosures of which are incorporated herein by reference. In accordance therewith, seals are made directly between portions of the incandescent filament and the quartz envelope by keeping the diameter of the tungsten filament wire at a value less than 0.004 inch, by outgassing the filament wire prior to and during sealing to preclude the presence of any adsorbed gas and by continuously flushing the filament and the quartz during sealing to preclude the presence of any molecular or gaseous impurties evolved from either during heating to be interposed between the quartz and the tungsten filament in the final seal. After the quartz has been collapsed or shrunk directly over end portions of the filament, the terminals or lead-in wires are attached externally by a brazing technique.

It may be here noted that in some cases in the past, the ends of the coiled tungsten filament in tubular lamps have been pulled out to form straightened leg portions which extended into the pinch seals at respective ends of the quartz lamp envelope where they were welded to foil portions of lead-in conductors. In such cases, the said foil portions constituted the usual hermetic seal with the quartz. Therefore, no attempt was made in such cases to hermetically seal the embedded portion of the filament, nor was a hermetic seal obtained inasmuch as the filament wire was usually of too large a diameter and, moreover, no particular precautions were taken to insure a hermetic seal in accordance with the disclosure of the aforesaid copend-ing applications of Millikan and Millikan et al. Actually, the straightened filament leg structure was employed to provide an internal fuse effect, as in Patent 3,211,942 to E. H. Wiley, or in some cases simply as an economy measure whereby the usual relatively heavy inner lead-in wire could be eliminated because the coiled filament was otherwise adequately supported.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an improved construction and method of assembly of the direct filament seal which will retain the advantages associated with elimination of the foil but which will also eliminate the problem of attaching the lead-in conductors externally to the sealed-in filament.

In accordance with the invention, the objects are achieved by preliminarily forming an assembly of the fine tungsten filament wire helically coiled on a mandrel of refractory metal other than tungsten, preserving the mandrel in the ends of the filament coil by coating those portions with an acid resistant material prior to dissolving out those portions of the mandrel between said ends to leave relatively massive legs which are arranged to project from the quartz lamp envelope when the filament is sealed therein and thereby serve as non-incandescible lead-in conductors, with the mandrel wire serving as a heat sink to avoid destructive overheating of the portion of the filament wire which is external to and extends beyond the hermetically sealed portion.

BRIEF DESCRIPTION OF THE DRAWING In the drawing,

FIG. 1 is an elevation of a lamp comprising the invention;

FIG. 2 is a fragmentary elevation, in section and on a larger scale, of the lamp of FIG. 1;

FIG. 3' is a fragmentary elevation, on a still larger scale, of a coiled filament and mandrel assembly during the process of lamp manufacture, the coil being sectioned to clearly show the mandrel;

FIG. 4 is an elevation of an assembly of a compact lamp unit in an outer bulb; and

FIG. is a fragmentary elevation of a portion of a lamp unit having a modified species of support construction.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1 of the drawing, the lamp illustrated therein is of the tubular double-ended type comprising a tubular envelope 1 of essentially fused silica which may be a vitreous quartz or a material sometimes referred to as 96% silica glass or Vycor, and which has a pinch seal portion 2 at each end. The envelope 1 contains a filament 3 of tungsten wire of not greater than 0.004 inch diameter and herein illustrated as embodying a coiled-coil body portion 4 extending axially of the envelope. The filament has longitudinally extending helically single-coiled end portions 5 which extend into the respective pinch seals 2 where there is a section 6 of quite large or open pitch, followed by a leg 7 which is shown as of normal pitch but which may also be of open pitch and which contains a mandrel wire 8 and which extends from within the pinch seal 2 to the exterior. The filament portions 6 of open pitch are hermetically sealed directly to the quartz or fused silica of the pinch seals 2. The relatively massive leg assembly 7 of filament and mandrel wire has its inner end embedded in, but not hermetically sealed to, the material of the pinch seal 2, and it is sufficiently massive to constitute a non-incandescible lead-in conductor which is not subject to destructive overheating.

By way of specific example only, the lamp of FIG. 1 may be of 100 watt size for operation at about 120 volts, with a tungsten wire filament of about 0.0025 inch diameter and an envelope 1 of about A inch diameter and an overall length, including the seals 2, of approximately 1% inch. The envelope may be filled with an inert gas such as nitrogen, argon, krypton, xenon or mixtures thereof, and a small quantity of halogen such as iodine or bromine or a compound thereof such as hydrogen bromide or a hydrocarbon compound of bromine.

In the manufacture of the lamp, the filament 3 is formed by helically coiling the tungsten wire tightly around the mandrel 8 which is of a refractory metal wire other than tungsten and which has a higher melting point than the softening temperature of the quartz envelope material. Molybdenum is preferred for the mandrel 8, although other metals might be used such as tantalum or platinum or alloys of the refractory metals. As shown in FIG. 1, the area of the primary coil at 6 which is to be hermetically sealed in the quartz, is preferably wound at a quite open pitch, for example about 40 turns per inch. In this case, the main body portion 4 of the filament is again helically coiled in known manner about a removable secondary mandrel.

The primary mandrel 8 is then removed except for the portions within the end legs 7. This may be performed in a manner known per se wherein the said end legs 7 are coated with an acid-resinous material, and the portion of the mandrel 8 between said ends is dissolved by immersing the entire filament assembly in a suitable acid in which the mandrel metal is soluble but not the tungsten. The coating of acid-resistant material is then removed to leave a relatively massive leg 7 required to withstand the sealing conditions and to serve as a lead-in conductor. The acid-resistant material may be a Wax or resin of high melting point. Suitable materials are those known as Picco resins made by Pennsylvania Industrial Chemical Cor-poration of Clairton, Pa.; one designated as No. 450 is an indene polymer and another designated No. 480K is a styrenated coumarone-indene type resin. These'resins are dissolved in trichloroet hylene to form a syrup which is applied to the coil legs and dried :by solvent evaporation prior to dissolving the mandrel. A mandrel of molybdenum may be dissolved in a mixture of equal proportions, by volume, of nitric and sulfuric acids.

The filament 3 and envelope 1 may be assemblied and sealed together on equipment somewhat similar to that, for example, shown in Patent 2,855,265 to Foote et al. The cylindrical envelope 1, open at both ends, is supported in a vertical position through a laterally extending quartz exhaust tube, the tipped-off residue of which is shown at 10 in FIG. 1.

The filament 3 is supported vertically and axially of the envelope 1 by gripping the upper leg 7 in a cap member which rests upon and closes off the upper end of the envelope, and the lower leg 7 is pulled down to slightly stretch the filament coil and is gripped in a suitable holder or chuck. A flow of inert gas, such as argon, is then started through the exhaust tube 10 and through the envelope 1 to purge the envelope of air and to provide a protective non-oxidizing atmosphere about the filament, the gas escaping from the lower end of the envelope which is preferably spaced only a slight distance from the surface of the chuck in which the lower leg 7 is held. The lower end of the envelope 1 is then heated by a pair of burners to temperatures above the softening point of the quartz envelope. The lower end of the filament, particularly the portion 6 is heated to a white heat (for example between about 1750 to 2200 C.) by radiation, conduction and convection of heat from the extremely hot envelope and the highly heated gas flowing through the envelope, so that the said filament portion to be sealed is kept clean and free of oxide and adsorbed gases.

The plastic lower end of the envelope is then pinched by a suitable pair of jaws to flatten it and hermetically seal therein the open pitch portion 6 of the filament and to also embed therein the inner end of the leg 7. Immediately upon pinching of the envelope, a flow of inert gas such as nitrogen may be directed at the exposed outer end of the leg 7 to prevent oxidation until the seal 2 has cooled down.

The envelope is then inverted, the aforesaid cap is removed while the flow of argon through the envelope is continued, and the then lower end of the envelope is fused and pinch sealed in the same manner as described above.

Subsequently, the envelope 1 may be exhausted, and the final fill gas and halogen added in known manner after which the exhaust tube 10 is sealed or tipped off. The envelope may be filled with argon or krypton at a pressure of several thousand torr, preferably with a small amount (say 5 to 20%) of nitrogen to avoid arcing across the filament when lighted, and a small amount of iodine may be added.

A lamp shown in FIG. 1 may, if desired, have pinch seals 2 of I-shaped cross section and may be provided with bases at respective ends thereof as shown, for example, in Patent 3,001,096 to F. A. Mosby. The base construction may serve to reinforce the lead-in conductor 7 and to provide a heat sink effect in conjunction with fixture sockets.

The small compact lamp of FIG. 1 may be mounted within an outer bulb in place of the simple filament of conventional incandescent lamps to provide a lamp of exceptionally high efficiency and maintenance of light output for a long, useful life. Such a lamp is shown in FIG. 4 where corresponding parts are numbered the same as in FIG. 1 with the addition of the letter a. The lamp 1a is like that of FIG. 1 in all respects except that in this case the pinch seals 2a are simply flattened throughout for their full width, and each leg or lead-in wire combination of filament and mandrel wire 7a is increased in length and doubled back upon itself with its terminal end also embedded in the pinch seal 2a. This structure may be desired or needed to provide additional strength where the leg 7a is of comparatively fine wire and is used to support the lamp unit In on respective supporting current supply or lead wires 11 and 12 which have portions thereof sealed in the pressed seal portion 13 of a conventional glass stem 14 which has its lower flared end sealed to a glass bulb 15. The bulb 15 may be evacuated and filled with an inert gas such as nitrogen through a conventional exhaust tube 16. The enclosing of the lamp unit 1a in the bulb 15 prevents oxidation of the legs or lead-in conductors 7a including those portions which are embedded in the pinch seals 2a but are not hermetically sealed therein because of their relatively large size. It will be understood that in the finished lamp the bulb 15 is provided with a base such as the conventional screw base for insertion in a conventional socket.

In FIG. 5, where parts corresponding to those in FIGS. 1 and 4 are again numbered with the same numeral plus the letter b, additional strength for supporting the lamp unit 1b from the support and current supply conductor 11b is provided by a spud wire 18 which has one end embedded in the pinch seal 2b. The spud 18 is of larger diameter wire than the leg or lead-in 7b, and both the spud 18 and leg 7b are clamped to the current supply and support wire 11b. It will be understood that a spud similar to 18 is also affixed to the other end of the lamp unit 1b. The inner end 19 of spud 18 may be deformed, for example by flattening it, in order to firmly anchor it in the seal 2b.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An incandescent electric lamp comprising a hermetically sealed envelope of essentially fused silica, an incandescible filament of tungsten wire of a diameter not greater than about 0.004 inch supported within said envelope and having a unitary integral end portion extending completely through and emerging exteriorly of a compressed seal portion of the envelope, an intermediate seal portion of said filament wire which extends from the inner end of said envelope seal portion to a point adjacent to but terminating short of the outer end of said envelope seal portion being hermetically sealed therein by direct intimate contact, the portion of said filament wire which extends outwardly beyond said seal portion from within the envelope seal portion to the exterior being helically coiled and containing a close fitting mandrel of refractory metal wire which serves as a heat sink whereby the combined said helically coiled filament wire and enclosed mandrel constitute a non-incandescible lead-in conductor.

2. A lamp as in claim 1 wherein the portion of said lead-in conductor at the exterior of the said seal portion is doubled back upon itself and has its extremity also embedded in said seal portion.

3. A lamp as set forth in claim 1, and further including a spud wire having one end also embedded in said seal portion and extending to the exterior thereof as a support member for the lamp.

4. In the manufacture of an incandescent lamp comprising a hermetically sealed envelope of essentially fused silica and an incandescible filament of tungsten wire of of a diameter not greater than about 0.004 inch supported within said envelope and having a portion at each end thereof hermetically sealed directly to and within a compressed seal portion of the envelope, the method which comprises forming a helical coil of the tungsten filament wire about a mandrel of wire of a refractory metal other than tungsten and having a melting point above the softening temperature of the fused silica, coating the respective end portions of the coil and mandrel assembly with an acid-resistant material, dissolving out the mandrel between the said end portions to leave said end portions as relatively massive legs of coil and enclosed mandrel wire, removing the coating material from said legs, disposing the filament within the envelope with a leg extending from a point within an open end portion of the envelope to the exterior, heating the said end portion of the envelope above the softening point thereof and simultaneously heating the portion of the filament contiguous to the leg while maintaining an inert gas atmosphere around the filament to outgas the surface thereof, and compressing the said end portion of the envelope to form a hermetic seal with the heated said portion of the filament and to embed and anchor therein the part of the leg portion located within the said end portion of the envelope.

References Cited UNITED STATES PATENTS 3,270,238 8/1966 Mosby 313279 JAMES W. LAWRENCE, Primary Examiner.

R. F. HOSSFELD, Assistant Examiner.

U.S. Cl. X.R. 29-25.13; 17450.6; 313-222, 331 

